In recent years, manufactures of Si semiconductor devices have moved to further finer design rules and have started commercial production of 65-nm CMOS devices. Since transistors manufactured with the finer design rules of CMOS technology can be used at higher frequencies, the manufactures have been promoting research and development of the technology for application to devices which operate at submillimeter waves or millimeter waves, such as an in-vehicle radar or a wireless HDMI system.
Circuits for operation at radio waves in a higher-frequency range such as submillimeter waves or millimeter waves often have a structure for differential signaling so that high tolerance for noise and stable gain can be achieved. On the other hand, in a module including a semiconductor IC, an antenna for transmission and receiving of signals has a structure in which the signals are transmitted through a single line. This structure is intended for reduction in complexity and miniaturization. Here, a balanced-unbalanced transformer which converts between a balanced signal and an unbalanced signal, that is, a balun is necessary for connecting lines for the balanced signal (balanced transmission line) inside a semiconductor IC and a single line (unbalanced transmission line) of an antenna.
There are two types of baluns: one is an active balun including a transistor, and the other is a passive balun including transmission lines. In active baluns, the higher the frequency of a signal, the larger the phase shift of the signal. Furthermore, noise from the transistor included in the balun degrades noise characteristics of the whole system, so that the balun is more likely to be affected by distortion. In contrast, since passive baluns do not include active elements such as a transistor, phase shift in signals is small so that such degradation of noise characteristics and distortion characteristics can be avoided.
Marchand baluns are commonly used passive baluns. A Marchand balun includes unbalanced transmission line and balanced transmission lines. The unbalanced transmission line and balanced transmission lines are not connected so that a direct current cannot flow therebetween. However, signals are transmitted between the unbalanced transmission line and the balanced transmission lines via electro-magnetic coupling between coupled lines as illustrated in FIG. 14A. In a Marchand balun, one unbalanced transmission line is provided with two balanced transmission lines having a dielectric layer between the unbalanced transmission line and the balanced transmission lines so that the same electro-magnetic coupling occurs between the unbalanced transmission line and each of the balanced transmission lines. A basic Marchand balun 100 includes an unbalanced transmission line 101, a first balanced transmission line 102, a second balanced transmission line 103, a single input-output terminal 104, and balanced input-output terminals 105a and 105b, and a dielectric layer 106. An end portion of the unbalanced transmission line 101 not provided with the single input-output terminal 104 is grounded. An end portion of the balanced transmission line 102 not provided with the balanced input-output terminal 105a is also grounded. Similarly, an end portion of the balanced transmission line 103 not provided with the balanced input-output terminal 105b is also grounded. Capacitors 107a, 107b, and 107c for blocking a direct current (DC) are provided between these end portions and a ground layer. One of characteristics of the Marchand balun 100 is its small size compared to a rat-race coupler 200 illustrated in FIG. 14B, which is of another type of baluns. The unbalanced transmission line 101 included in the Marchand balun 100 has a length of (λ/2) μm, and the balanced transmission lines 102 and 103 has a length of (λ/4) μm.
The Marchand balun 100 can be made in a smaller size for use with a higher frequency. However, each of the balanced transmission lines 102 and 103 for a frequency as high as 60 GHz still need to be approximately 600 μm long, (on a Si semiconductor substrate such as a CMOS). As such, this has been a problem with further miniaturization of Marchand baluns.
A balun 110 is an example of conventional technique to achieve miniaturization of baluns. The miniaturization has been achieved by providing a capacitor 108 between the balanced outputs of the balun 110 as illustrated in FIG. 15. Hereinafter, a balun including only coupled lines is referred to as a Marchand balun, and a balun including a capacitor provided between its balanced input-output terminals is generically referred to as a balun. In FIG. 15, the components also shown in FIG. 14A are denoted with the same reference signs, and therefore a description thereof is omitted. The capacitor 108 connects the balanced input-output terminals 105a and 105b. A capacitor 109 is provided between a single input-output terminal and a ground layer. The capacitor 108 between the balanced input-output terminal 105a and 105b introduces phase lead so that the balanced transmission lines 102 and 103 included in the balun 110 may be (λ/4) μm or shorter. For a balun for the 60-GHz band, the balanced transmission lines 102 and 103 may be shortened to 100 to 200 μm long (on a Si semiconductor substrate such as a CMOS). Although there is a problem that the introduction of phase lead by the capacitor makes the bandwidth of the balun narrower, the balun still has a bandwidth of approximately 10 to 15 GHz. In the ultra wide band (UWB) such as the 24-GHz band or the 60 GHz band, the balun can be used in a bandwidth of approximately 7 GHz, and an occupied bandwidth in the 60 GHz band extends across approximately 500 MHz, which is sufficiently wide.